Liquid crystal color switch and method of manufacture

ABSTRACT

This invention involves one kind of liquid crystal color light switch, made especially from cholesteric liquid crystals. By changing the voltage applied, the reflection wavelength of the color light switch can be controlled, leading to a rapid change of the reflected light from red to green to blue in rapid succession, thus creating a color display. The core technology of this invention is making the voltage applied to the cholesteric liquid crystal less than the untwisted voltage and still in good focal conic state, that is the symmetric axis are in the same direction, and the pitch is varied according to the voltage applied.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is for the same invention and is claiming thefiling date of Chinese application #02128839.9 filed on Aug. 15, 2002,entitled ______.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] Not Applicable

BACKGROUND OF THE INVENTION

[0003] People continue to demand better and better quality for low-powercolor liquid crystal displays. In some special applications, for examplefor large-screen LCOS (Liquid Crystal On Silicon) projection displays,the requirements include photo-realistic images, fast and accuratecolors, and high light efficiency. So far, no color light switch existsthat can accept strong light illumination and absorb close to zero lightin the process.

[0004] Currently, two major technologies dominate the use of liquidcrystal displays for displaying color. The most common is STN-LCD andTFT-LCD, which use color filters to determine its basic colors thatcannot be changed by external means. Another is Electrically ControlledBirefringence (ECB) liquid crystal display technology, where the basiccolors are obtained by controlling the voltages applied to the liquidcrystal layer to change the retardation. The light mostly concentratesin the center, thus it is not as clear around the edges, and lightefficiency is low. Neither of these technologies can change colorsquickly. Also, both technologies use polarizers and color filters thatabsorb light. When strong light is shone on the imagers used forprojection displays, the temperature will rise to a point where itexceeds the normal operating range of the imager. Thus, color displaysbased on current liquid crystal technologies cannot fulfill the needs ofcolor projection.

BRIEF SUMMARY OF THE INVENTION

[0005] This invention concerns a color light switch and the method ofits manufacture. Through the control of an applied voltage, the colorlight switch can change its critical reflection wavelength, so that theincident white light from an oblique angle will reflect red and thengreen and then blue in rapid succession in response to the appliedvoltage, thus realizing a color display system.

[0006] It is an object of the invention to provide a liquid crystalcolor light switch, with a cell of liquid-crystals, comprising upper andlower transparent substrates, sealed on all sides, with correspondinginside surfaces of the transparent substrates coated with transparentelectrodes. The inside surface of the transparent electrodes are furthercoated with alignment layers, with the pre-tilt angle ranging from about1° to about 89°. A layer of cholesteric liquid crystals in a focal conicstate is between the two transparent electrodes, with symmetrical axisof all domains in the same direction, parallel to the two transparentelectrodes. The controlled voltage is applied to change the criticalreflection wavelength.

[0007] It is a further object of the invention to provide a method ofmanufacturing a liquid crystal color light switch with liquid crystal ina cholesteric phase, being a mixture of nematic liquid crystal andchiral components, in weight proportion of about 0.67:1 to about 10:1.

[0008] On the upper and lower substrates are evaporated transparentelectrodes, further coated with organic material or evaporated withinorganic material as alignment layers. The prepared liquid crystal cellis placed inside an oven, heated above the clean point of the liquidcrystal mixture, and then cooled slowly, until a good one-dimensionalfocal conic state is reached, the temperature of clean point is 70° C.to 100° C.

[0009] Normal liquid crystal cell is filled with cholesteric liquidcrystals containing both planar and focal conic states. In the planarstate, the symmetric axis is perpendicular or near perpendicular to thesubstrate. When light is incident, only that with same rotationdirection as cholesteric liquid crystal structure will produce Braggreflection, while the rest is transmitted out of the liquid crystallayer.

[0010] In the quiescent state, with no applied voltage, the wavelength λof the reflected light and bandwidth Δλ are respectively:

λ=nP cosθ  (1)

Δλ=Δn/nλcosθ or Δλ=Δn P cosθ  (2)

n=(n_(e)+n₀)/2   (3)

[0011]n is the average refractive index of cholesteric liquid crystal. Pis twist pitch. Θ is the angle between the incident light and symmetryaxis. Δn=n_(e)−n₀. n_(e) is extra ordinary refractive index of liquidcrystal. n₀ is ordinary refractive index .

[0012] In the normal focal conic state, the symmetric axis is parallelor near parallel to the substrate, and the domains are two-dimensionalrandomly distributed. When the incident light is shone, a small amountis scattered back, while the rest is transmitted through the device.

[0013] In the quiescent state, there are two different structures/statesand a lot of gray scale middle states, all of which are stable. Thereare numerous small domains with their symmetry axis facing differentdirections. At a certain electric field, the two different states canchange back and forth in the same phase.

[0014] This invention utilizes special surface treatments and drivetechnology to obtain within the total or partial liquid crystal layer astable one-dimension aligned focal conic state. The incident light andreflected light are on opposite sides of the liquid crystal cell.

[0015] According to formula (1), the critical reflection wavelength isdetermined by the pitch P. the incident angle theta, and the averagerefractive index of the liquid crystal material.

[0016] The pitch P of the cholesteric liquid crystal can be modified bychanging the applied voltage. In other words, P is a function of V. Whenthe applied electromagnetic field is strong enough, the pitch P becomesinfinitely long.

[0017] This liquid crystal color light switch operates when P is withinlimited values so that the liquid crystal maintains its focal conicstate and no planar state appears. At a different electromagnetic field,the corresponding pitch P(V) corresponds to different reflectionwavelength. When the reflection wavelength is inside visible lightspectrum, it will correspond to different colors. As a result, thecenter of the reflection wavelength is continuously adjustable.

[0018] This invention possesses the following advantages over existingtechnologies:

[0019] 1. This invention uses the electro-optic characteristics ofcholesteric liquid crystals, using the applied voltage on the liquidcrystal layer to change the wavelength of the reflected light, so thatthe voltage can rapidly change the colors being reflected.

[0020] 2. This invention is not constructed with layers that absorblight (e.g., polarizers, color filters), which would raise the operatingtemperature of the device under strong light illumination, and whichwould reduce the light efficiency of the device. This device allows useof strong light illumination, and is suitable for use with LCOS, DMD andother imagers and other telecommunications devices.

[0021] 3. This invention produces very little scattering of light. Thetransmitted and reflected lights travel without scattering effects.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 depicts a diagram of the invention (color light switch).

[0023]FIG. 2 depicts the interaction of light with the color lightswitch.

[0024]FIG. 3 depicts the color spectrum of the reflective light underthe various control voltages applied.

DETAILED DESCRIPTION OF THE INVENTION

[0025]FIG. 1 illustrates an example of a liquid crystal color lightswitch. In this example, we have a liquid-crystal cell, consisted ofupper and lower transparent substrates (101), made of glass or othermaterials, sealed on all sides (102), with corresponding inner surfacesof the transparent substrates coated with transparent electrodes (103),transparent conductive coatings are typically made with ITO (Indium-TinOxide, which is Indium oxide doped with tin oxide), the inside surfaceof the transparent electrodes coated with alignment layers (104), withthe pre-tilt angle ranging from 1° to 89°, a higher pre-tilt angle(>30°) produces more stable focal conic state, the alignment layer helpsalign the liquid crystals with a correct alignment and pre-tilt angle.In this example the pre-tilt angle is around 60°.

[0026] There is a layer of cholesteric liquid crystals (105) between thetwo transparent substrates, the liquid crystals are in one dimensionaligned focal conic state, the symmetrical axis (106) of all domains arein the same direction, and parallel to the two transparent substrates.The applied voltage is controlled to change the critical reflectionwavelength. The cholesteric liquid crystal material used is a mixture ofnematic liquid crystal and chiral component, their proportion by weightbeing 2:3 to 10:1. The best ratio is around 1:1 to 4:1. Other inactiveingredients, such as coagulants and surfactants, can also be added, notexceeding 5% by weight of the mixture.

[0027] The key to a successful liquid crystal color light switch is theconstruction of the liquid crystal cell and the selection of the liquidcrystal mixture. The thickness of the liquid crystal should be small. Inour example, the thickness is 1 to 10 micrometer. The liquid crystalcell measures 0.8″ by 0.8″, without use of spacer.

P=1/ΣA_(i)C_(I)

[0028] A_(i) Is the twist power of No.i twist component; C_(I) is theconcentration of the component.

[0029] In FIG. 2, this invention is integrated with two supportingtrapezoidal prisms (200, 202). To simplify manufacturing to suit massproduction, it is best to use material with compatible refractiveindices, like Canadian Balsam or cured resin, to glue the transparentprisms to the liquid crystal cell substrates. Inside the liquid crystalcell is the one dimension aligned cholesteric liquid crystal material(201) in focal conic state, its symmetric axis (206) being parallel toone another and to the substrates. Incident light (203) and reflectedlight (205) are on opposite sides of the liquid crystal cell, and in thesame plane with the symmetric axis. If we view the incident white lightas consisted of left and right circularly polarized, only the light withthe same polarity as the cholesteric liquid crystal at a certainwavelength will be reflected, the rest will be transmitted, forming thetransmitted light (204). The transmitted light is the complement of thereflected light, which is the incident light minus the reflected light.

[0030] The center wavelength of the reflected light is:

λ=nP cosθ

[0031] In FIG. 3, when a voltage of V₀=0 is applied to the liquidcrystal cell, curve 301 shows the reflected light spectrum is centeredat wavelength of 400 nm. When a voltage of V₁ is applied to the liquidcrystal cell, curve 302 shows the reflected light spectrum is centeredat wavelength of 450 nm. When a voltage of V₂ is applied to the liquidcrystal cell, curve 303 shows the reflected light spectrum is centeredat wavelength of 550 nm. When a voltage of V₃ is applied to the liquidcrystal cell, curve 304 shows the reflected light spectrum is centeredat wavelength of 610 nm.

[0032] In this example, the applied voltage is a pulsed voltage thatdoes not have direct current component when averaged over time.

Example 1 of the Method Used in Manufacturing the Color Light Switch

[0033] a. Preparing the liquid crystal mixture using EM suppliedchemicals. The proportion by weight ofZLI-5400-100:R1011:CB-15:R-811=76:5:12:7.

[0034] b. Depositing on the upper and lower substrates thin film layersof ITO by evaporation to form transparent electrodes, on top of whichare deposited SiOx, the pre-tilt angles being around 60°.

[0035] c. The pre-tilt angles of the alignment layer on the upper andlower electrodes are parallel but in opposite direction.

[0036] d. Construct a liquid crystal cell with thickness of 2.3micrometer, and size of 0.8″ by 0.8″. Through vacuum suction orcapillary action, the cell is filled with the prepared liquid crystalmixture, and all sides sealed with sealant;

[0037] e. The outer surfaces of the upper and lower glass substrates arecleaned thoroughly, Canadian balsam applied, and two trapezoidal prismsglued on the cleaned surfaces, with care taken to ensure that no bubblesare trapped in the process;

[0038] f. The prepared liquid crystal cell is placed inside an oven,heated at or above the clean point of the liquid crystal mixture, andthen cooled slowly, until a good single-domain focal conic state isreached, the corresponding temperature is 70° C. to 100° C. (80° beingbest.)

Example 2 of the Method Used in Manufacturing the Color Light Switch

[0039] a. Preparing the liquid crystal mixture using EM suppliedchemicals. The proportion by weight ofZLI-5400-100:R1011:CB-15:R-811=76:5:12:7.

[0040] b. Evaporating on the transparent electrodes on the upper andlower glass substrates is SiOx, with pre-tilt angle of 65°;

[0041] c. Rubbing in a single direction applied opposite to the organicalignment material on the surface;

[0042] d.Through vacuum suction or capillary action, filling the cellwith the prepared liquid crystal mixture, and sealing all sides;

[0043] e.Cleaning the outer surfaces of the upper and lower glasssubstrates thoroughly, applying Canadian balsam, and gluing twotrapezoidal prisms glued on the cleaned surfaces, with care taken toensure that no bubbles are trapped in the process; and

[0044] f. Applying a voltage greater than the untwist voltage betweenthe two substrates, the alternating frequency being 1 kHz, the voltagebeing around 25V, and lasting longer than 10 microseconds, and then thevoltage dropped gradually to 10V in about 2 seconds or longer, to enablethe liquid crystals to get into field induced nematic state, and thenslowly reduce the voltage further, until a good one-dimensional focalconic state is obtained.

What is claimed is:
 1. A liquid-crystal color light switch, comprising:a cell of liquid-crystals with upper and lower transparent substratessealed on all sides, wherein; corresponding inner surfaces of said upperand lower transparent substrates are coated with transparent electrodesand alignment layers having a pre-tilt angle ranging from 0 to 90°; anda layer of cholesteric liquid crystal in a focal conic state iscontained between said transparent electrodes; said cholesteric liquidcrystal having the symmetrical axes of all domains in the samedirection, parallel to the two transparent electrodes; and containingprovision that a controlled voltage may be applied to said transparentelectrodes to change the critical reflection wavelength.
 2. Aliquid-crystal color light switch of claim 1, wherein the thickness ofthe liquid-crystal cell is 1-10 micrometers without a spacer.
 3. Aliquid-crystal color light switch of claim 2, further comprising: liquidcrystal in a cholesteric phase, being a mixture of nematic liquidcrystal and chiral components, their proportion by weight being fromabout 0.67:1 to 10:1.
 4. A liquid-crystal color light switch of claim 2,further including: the pre-tilt angle being 5-65°.
 5. A method ofmanufacturing a liquid crystal color light switch, comprising: a.preparing the liquid crystal mixture of cholesteric phase containing thedesired proportion of nematic and chiral components between 0.67:1 and10:1, b. on the upper and lower substrates, evaporating transparentelectrodes, c. further coating the transparent electrodes with organicmaterial or evaporating with inorganic material like SiOx as alignmentlayers; d. rubbing organic material on the surface in a single directionand the rubbing direction of upper and lower glass are parallel eachother. e. filling the cell through vacuum suction or capillary actionwith the prepared liquid crystal mixture and sealing on all sides, f.cleaning the outer surfaces of the upper and lower glass substratesthoroughly, and gluing two trapezoidal prisms on the cleaned surfaces,taking care to ensure that no bubbles are formed in the process; g.placing the prepared liquid crystal cell inside an oven, heating abovethe clean point(a temperature of 70° C. to 100° C.) of the liquidcrystal mixture, and then cooling slowly, until a good one-dimensionalfocal conic state is reached.
 6. A method of manufacturing a liquidcrystal color light switch, comprising: a. Preparing the liquid crystalmixture of cholesteric phase containing the desired proportion ofnematic and chiral components between 0.67:1 and 10:1, b. on the upperand lower substrates, evaporating transparent electrodes, c. furthercoating the transparent electrodes with organic material or evaporatingwith inorganic material like SiOx as alignment layers having a pretiltangle of 5-65° d. rubbing the organic material on the surface in asingle direction and the rubbing direction of upper and lower glass areparallel each other. e. filling the cell through vacuum suction orcapillary action with the prepared liquid crystal mixture and sealing,e. cleaning The outer surfaces of the upper and lower glass substratesthoroughly, and gluing two trapezoidal prisms on the cleaned surfaces,taking care to ensure that no bubbles are formed in the process; f.applying a voltage greater than the untwist voltage between the twosubstrates, to enable the liquid crystals to get into field inducednematic phase, and then slowly reducing the voltage, until a goodone-dimensional focal conic state is obtained.